A conventional article of athletic footwear includes two primary elements, an upper and a sole structure. The upper provides a covering for the foot that securely receives and positions the foot with respect to the sole structure. In addition, the upper may have a configuration that protects the foot and provides ventilation, thereby cooling the foot and removing perspiration. The sole structure is secured to a lower surface of the upper and is generally positioned between the foot and the ground. In addition to attenuating ground reaction forces (i.e., imparting cushioning), the sole structure may provide traction and control foot motions, such as pronation. Accordingly, the upper and the sole structure operate cooperatively to provide a comfortable structure that is suited for a variety of ambulatory activities, such as walking and running.
The sole structure of athletic footwear generally exhibits a layered configuration that includes a comfort-enhancing insole, a resilient midsole formed from a polymer foam material, and a ground-contacting outsole that provides both abrasion-resistance and traction. In some articles of footwear, the midsole is the primary sole structure element that imparts cushioning and controls foot motions. Suitable polymer foam materials for the midsole include ethylvinylacetate or polyurethane that compress resiliently under an applied load to attenuate ground reaction forces. Conventional polymer foam materials are resiliently compressible, in part, due to the inclusion of a plurality of open or closed cells that define an inner volume substantially displaced by gas. The polymer foam materials of the midsole may also absorb energy when compressed during ambulatory activities.
The midsole may be formed from a unitary element of polymer foam that extends throughout the length and width of the footwear. With the exception of a thickness differential between the heel and forefoot areas of the footwear, such a midsole exhibits substantially uniform properties in each area of the sole structure. In order to vary the properties of midsole, some conventional midsoles incorporate dual-density polymer foams. More particularly, a lateral side of the midsole may be formed from a first foam material, and the medial side of the midsole may be formed from a second, less-compressible foam material. Another manner of varying the properties of the midsole involves the use of stability devices that resist pronation. Examples of stability devices include U.S. Pat. Nos. 4,255,877 to Bowerman; 4,288,929 to Norton et al.; 4,354,318 to Frederick et al.; 4,364,188 to Turner et al.; 4,364,189 to Bates; and 5,247,742 to Kilgore et al.
Another manner of varying the properties of the midsole involves the use of fluid-filled bladders. U.S. Pat. No. 4,183,156 to Rudy, discloses an inflatable insert formed of elastomeric materials. The insert includes a plurality of tubular chambers that extend substantially longitudinally throughout the length of the footwear. The chambers are in fluid communication with each other and jointly extend across the width of the footwear. U.S. Pat. No. 4,219,945 to Rudy discloses an inflated insert encapsulated in a polymer foam material. The combination of the insert and the encapsulating polymer foam material functions as the midsole. Examples of additional fluid-filled bladders for footwear include U.S. Pat. Nos. 4,906,502 and 5,083,361, both to Rudy, and U.S. Pat. Nos. 5,993,585 and 6,119,371, both to Goodwin et al.